Pull-in and drop-out delay unit for vehicle detector in traffic-control system

ABSTRACT

A pull-in and drop-out delay unit for a vehicle detector acting in conjunction with a traffic-light controller installed at a traffic intersection. The detector serves to sense the presence or absence of a vehicle in a zone adjacent the traffic intersection. When the detector senses the presence of a vehicle, the delay unit serves to defer the transfer of the presence signal to the traffic controller for an adjustable period to permit right-turn on &#39;&#39;&#39;&#39;red.&#39;&#39;&#39;&#39; When the detector senses the absence of a vehicle, the delay unit defers the transfer of the absence signal to the controller for an adjustable period whereby &#39;&#39;&#39;&#39;tailgating&#39;&#39;&#39;&#39; is minimized and slow-reacting vehicles are more easily passed.

United States Patent Friedman [4 1 Sept. 19, 1972 [54] PULL-IN AND DROP-OUT DELAY UNIT 2,130,013 9/1938 Hunter ..340/36 FOR VEHICLE DETECTOR 1N 2,090,619 8/1937 Biach et al ..340/36 TRAFFIC-CONTROL SYSTEM Primary ExaminerKathleen H. Claffy [72] Inventor Friedman Roslyn Assistant Examiner-Randall P. Myers [73] Assignee: Fischer & Porter Company, War- Attorney-Michael Ebert minster, Pa. [22] Filed: Oct. 21, 1970 L d d S I T f h l d pu m an rop-out e ay umt or a ve 1C e etec- [21] APP]- Noi 82,634 tor acting in conjunction with a traffic-light controller installed at a traffic intersection. The detector serves [52] US. Cl ..340/37 to Sense the pmsenc?or abseifce of a vehicle in a Zone 51 1nt.Cl. ..G08g 1/07 adjacent the "affic Intersection when the detector [58] Field of Search ..340/36 37 38 R 38 L Senses the Presence Of a, vehicle the delay serves to defer the transfer of the presence signal to the traf- 56 f fic controller for an adjustable period to permit right- I 1 Re erences cued turn on red. When the detector senses the absence UNlTED STATES PATENTS of a vehicle, the delay unit defers the transfer of the absence signal to the controller for an adjustable 3,602,882 8/1971 l-lill ..340/37 period whereby tailgating is minimized and slow 2; L reacting vehicles are more easily passed.

v u ivier 3,150,349 9/1964 Wilcox ..340/37 4 Claims, 2 Drawing Figures Burr/c J Cowpouee fqn /c 6 Q Q i /Z A t I Wow 4% Z 18 ury lY/T' flan-cm /5 v A v 56/76/06 loop I3 PULL-IN AND DROP-OUT DELAY UNIT FOR VEHICLE DETECTOR IN TRAFFIC-CONTROL SYSTEM RELATED APPLICATION This application is related to my copending patent application Ser. No. 29,393, filed Apr. 17, 1970, now US. Pat. No. 3,651,452.

BACKGROUND OF THE INVENTION This invention relates generally to vehicle detectors operating in conjunction with traffic controllers, and more particularly to a delay unit interposed between a vehicle detector and a traffic-light controller installed at a traffic intersection, which unit acts to improve the control characteristics of the system and to facilitate the flow of traffic.

In traffic-control systems in which the presence and absence of a vehicle at an intersection or other point of traffic control is sensed to carry out control functions, vehicle detection is generally effected by means of an inductive loop buried in the vehicle passageway or roadbed.

Thus in the TACDET vehicle detector manufacture by Fischer & Porter Co. of Warminster, Pennsylvania, and described in their 1969 Instruction Bulletin 3lLDl000, the buried inductive loop is connected to the input of the vehicle detector so that any size of metallic vehicle from a motorcycle to a tractor-trailer moving over the detection loop and lying within the effective zone thereof, will be detected.

Detection in a TACDET unit is based on the phase characteristic of a parallel-tuned resonant circuit whose inductance is constituted by the buried loop. This tuned circuit is driven by a radio-frequency carrier voltage whose amplitude andphase are constant. The voltage across the resonant circuit reflects the impedance thereof. The TACDET device exploits the change in phase of this impedance due to the presence of a vehicle, and acts to operate a relay which activates the controller associated with the detector.

There are other vehicle detectors functioning in conjunction with buried loops which operate on fixedfrequency or variable-frequency principles, such as those described in my co-pending application, aboveidentified. But in all instances, the presence of a vehicle is indicated by the actuation of a relay or by establishing a voltage whose level or polarity is indicative of vehicle presence or the absence thereof. The present invention isapplicable to all forms of vehicle detectors which indicate the presence or absence of avehicle lying within the operative zone of the loop.

In existing installations at traffic intersections, the moment a vehicle appears within the loop zone and is sensed, the loop detector delivers a presence signal to the controller for the traffic light to cause the light to turn green. The light reverts to red immediately after the vehicle departs from the loop zone. These fast and inflexible actions are disadvantageous, particularly in the case of short loops, for example those no larger than 6 by 6 feet.

One reason for this is that in localities permitting a right turn through a red light (i.e., California), it is not possible with existing systems for a car approaching an intersection to make a right turn without at the same time turning the traffic light green, for the moment the car enters the loop zone, the traffic light is caused to switch automatically from red to green. Since there may be no car in the lane following the car making the right turn, the resultant green signal in that lane is wasted, and is at the expense of green time in the crosslane at the intersection. Thus traffic flow in the cross lane may be needlessly disrupted.

Another reason why the existing systems do not facilitate the flow of traffic is that in the situation where two cars in the same lane approach an intersection with proper headway between the cars. the first car, when it enters the loop zone, will cause the light to turn green and will pass through the intersection. But the second car, even if it is merely 10 feet behind the first car, will not enter the loop zone (assuming a loop length of 6 feet) until after the first car has left the zone, in which case the traffic light will turn red before the second car enters the zone, forcing the second car to brake. But if the second car, in order to avoid coming to a halt, were to stay sufficiently close to the first car so that it entered the loop zone before the first car departed therefrom, this would result in objectionable tailgatmg.

Still another drawback of existing systems wherein the trafiic lights immediately switch phase upon the detection of a vehicle, is with respect to driven vehicles having a slow reaction time. When traffic lights change quickly upon detection of the presence or absence of a vehicle in the loop zone, a slowly-r eacting vehicle is not accorded sufficient time to take appropriate action. Thus existing fast-acting detectors are inflexible and fail in some instances to expedite the flow of traffic.

SUMMARY OF THE INVENTION In view of the foregoing, the main object of this invention is to provide an adjustable time-delay unit between a vehicle detector and a traffic controller to afford greater flexibility in the control of intersections, especially those provided with relatively short detection loops.

More specifically, it is an object of the invention to provide a delay unit of the above-noted type which introduces an adjustable delay period for deferring the transmission of a vehicle-presence signal to pull-in a traffic controller, and which also introduces an adjustable delay to defer the transmission of a vehicle-absence signal to drop-out the controller.

Among the advantages of the invention are that it makes it possible for a vehicle to turn right on a red signal without causing a changein the signal light, and it also permits cars approaching an intersection to maintain adequate headway and yet not cause a signal change interfering with the smooth passage of vehicles through an intersection.

Also an object of the invention is to provide a delay unit of the above-noted type which acts in the event of power failure or other operating defects to cause the traffic controller to receive a continuous presence" signal, thereby avoiding traffic blockage.

Briefly stated, these objects are accomplished in a preferred embodiment of a delay unit in accordance with the invention, the unit including a memory flip flop circuit having first and second cross-coupled transistors. The transistors are rendered alternately conductive by separate first and second control devices, one of which is activated when the vehicle detector senses the absence of a vehicle and the other when the detector senses the presence of a vehicle, each control device including an adjustable delay network.

A control relay in the output of the first transistor is coupled to a traffic controller and is arranged to normally maintain the light controlled thereby green, but when the first transistor is, after a predetermined delay introduced by the associated control device, rendered conductive by the absence of a vehicle, it acts to ener gize the relay to cause the light to turn red. And when the second transistor, after a predetermined delay introduced by the other control device, is rendered conductive by the presence of a vehicle, it acts to render the first transistor non-conductive to cause the light to revert to green. Since the control relay, in its static state, maintains a presence signal, should there be an electrical failure or a defect disabling the delay unit, the traffic controller will be supplied with a presence signal so that the delay unit in this condition does not block traffic flow. The reaction of the controller to the presence signal depends on the manner in which the controller is programmed, hence if a green light is activated, the period of activation depends on the controller action.

OUTLINE OF THE DRAWING For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawing, wherein:

FIG. 1 is a block diagram showing a vehicle detector in combination with a delay unit in accordance'with the invention, for operating a traffic controller; and

FIG. 2 is a schematic circuit diagram of a preferred embodiment of the delay unit.

BRIEF DESCRIPTION OF THE INVENTION Referring now to FIG. 1, there is shown a vehicle detector that senses the presence or absence of a vehicle C having a metallic body, by means of an inductive loop which is buried under the vehicle lane L. In practice, loop 10 may be in various dimensions, such as three coil turns in a 6by-6-feet loop or four coil turns in a 4-by-4-feet loop, depending on the desired zone of detection.

Instead of using an inductive loop, other forms of sensors sensitive to the presence of a vehicle may be used, such as a buried, pressure-sensitive capacitor whose electrode spacing is decreased by the presence of a vehicle to produce an impedance change.

Inductive loop 10 is coupled by leads 11 to a detector 12 provided with an output relay 13 having a call" contact A which is engaged only when the presence of a vehicle is sensed, the armature of the relay normally engaging a no-call contact B. The relay therefore acts as a single-pole, double-throw switch. Thus, when a vehicle enters the loop zone, call contact A switch is immediately closed, and when the vehicle departs from the zone, no-call contact B switch is closed.

In practice, an equivalent action may be effected by a solid-state device such as a device generating a voltage of one polarity in the presence of a vehicle and a voltage of reverse polarity in the absence thereof. Be

tector 12 may be of the TACDET type or of any other type previously mentioned, adapted to provide call and no-call" indications regarding the presence or absence of a vehicle in a detection zone.

Relay 13 is coupled to a pull-in and drop-out delay unit 14 in accordance with the invention, which unit is provided with an output relay 15 (or equivalent solidstate device) adapted to actuate a normally-closed switch S. Switch S is coupled to a traffic controller .I operating a traffic-light signal-box T at the intersection of lane L and a cr0sslane. The arrangement is such that when switch S is closed (its normal state), controller .I produces a green traffic signal, but when relay 15 is energized and switch S is opened, the controller causes the signal light to turn red. The function of the delay unit is to defer the transfer of the call and nocall indications from the detector to the controller for separately adjustable periods in order to accomplish the objects of the invention.

Referring now to FIG. 2, there is shown a preferred embodiment of the delay unit 14. The unit includes a memory flip-flop circuit provided with two main transistors 16 and 17. The main transistors are crosscoupled, the base of transistor 16 being coupled to the collector of transistor 17 through an R-C network 18 and the base of transistor 17 being coupled to the collector of transistor 16 through an R-C network 19. When one main transistor is in the ON or conductive state, the other is in the OFF or non-conductive state, and vice versa.

A control device CD is associated with main transistor 16, the device including a unijunction transistor 20 and a capacitor 21 connected between the emitter E and base B thereof. The unijunction transistor has electrical characteristics quite different from those of conventional two-junction transistors, its most important features being a stable triggering voltage which is a fixed fraction of the applied interbase voltage, and a very low value of firing current. A similar control device CD is associated with main transistor 17, this device including a unijunction transistor 22 and a capacitor 23. Each control device includes a delay network which acts to delay the point at which the associated main transistor is rendered non-conductive when the control device is activated.

Detector relay 13 contacts A and B are arranged so that when contact B is engaged, it shorts out capacitor 21 in device CD, through a current-limiting resistor 24, and when call" contact A is engaged, it shorts out capacitor 23 in device CD through the same resistor. These capacitors are charged from the d-c power source for the unit through variable resistors 25 and 26, respectively, the charging rate being determined by the resistor setting.

When capacitor 21 is charged to a given level, the voltage thereacross is sufficient to fire unijunction transistor 20 to render it highly conductive, thereby producing current flow in a resistor 27 connected in the circuit of the second base B The resultant negative pulse across resistor 27 is applied to the base of main transistor 16 through coupling capacitor 28, the pulse acting to turn off main transistor 16. This action causes main transistor 17 to flip on.

Similarly, when the charge on capacitor 23 reaches a given level, the voltage thereacross fires unijunction transistor 22 to render it highly conductive, thereby producing current flow in a resistor 29 in the circuit of the second base B The resultant negative pulse produced across resistor 29 is applied through coupling capacitor 30 to the base of main transistor 17, to turn off this transistor and to flip on main transistor 16.

Detector relay 13 serves therefore to short out either capacitor 21 or 23 depending on its position, thereby determining which timing circuit (CD or CD is active. Since variable resistors 25 and 26, respectively, control the charging rate of these capacitors, they function to determine the time of firing of unijunction transistors 20 and 22.

Output relay 15 is connected in the collector circuit of main transistor 16, such that when this transistor is in the OFF state, relay switch S, which is normally closed, is unaffected and remains closed so that the controller .1 associated therewith produces a green traffic light. When, however, main transistor 16 is turned on, relay 15 is energized to open switch S to cause the traffic light to turn red.

When no car is within the detection zone for a period of time which is longer than the time-out period, the detector relay 13 will be in the no-call contact B position, in which condition capacitor 21 is shorted out and unijunction transistor 20 is disabled. But at the same time capacitor 23 is connected to the open contact A; hence this capacitor is not shorted and charges at a rate determined by resistor 26, until a point is reached at which unijunction transistor 22 fires, thereby turning main transistor 17 off and main transistor 16 on. Thus in the no-call" state, output relay 15 is energized to break switch S and to cause the traffic light to turn red.

When detector relay 13 thereafter changes position due to the presence of a car within the loop zone, call contact A is engaged, thereby discharging capacitor 23, and since contact B is now disengaged, capacitor 21 proceeds to charge. After a time interval determined by the time constant of the delay network, unijunction transistor 20 is fired, thereby turning off main transistor 16and turning on-main transistor 17. As a consequence, relay 15 is de-energized, switch S reverts to its closed condition, and the traffic light turns green. Thus a pullin action is obtained with an adjustable delay.

After the car leaves the detection zone, the detector relay will return to the no-call position. However, output relay 15 remains de-energized until capacitor 23 has charged to a sufficient level to render unijunction transistor 22 conductive, which causes main transistor 17 to turn oi? and main transistor 16 to turn on, to again energize the relay and open switch S. Thus a drop-out action is obtained, with an adjustable delay.

A If a second car enters the loop zone prior to the charging of capacitor 23 to a sufficient level to fire unijunction transistor 17, then the detector relay will revert to call position A and discharge capacitor 23 through current-limiting resistor 24. The memory flipflop will, in this circumstance, not change state, for unijunction transistor 22 does not fire. Thus, even though the detector relay goes from call to no-call" in the interval between the spaced first and second cars travelling in the lane, the delay introduced by the unit prevents the controller from switching the traffic signal to red in this interval and makes possible uninterrupted travel with adequate headway.

Similarly, if the system is in a no-call state with the traffic-light red (main transistor 1.6 conductive), and a car enters the loop zone intending to make a right turn on red, the presence of the car would be detected and detector relay 13 would switch to call contact A to short out capacitor 23, at which point capacitor 21 would proceed to charge. But if the car left the loop zone to make a right turn on the red light before the charge of capacitor 21 attained a level sufficient to fire unijunction transistor 20, at the moment the car departed the loop zone, the detector relay would switch over to no-call contact B, thereby shorting out capacitor 21 and hence causing main transistor 16 to remain conductive to keep the signal on red. Thus the delay unit makes possible a right turn on a red signal without effecting a traffic-light change.

While there has been shown and described a preferred embodiment of pull-in and drop-out delay unit for vehicle detector in traffic-control system in accordance with the invention, it will be understood that many changes and modifications may be made therein without, however, departing from the essential spirit thereof.

I claim:

1. A traffic-control system comprising A. a traffic-controller adapted to operate green and red signals at an intersection of two streets,

B. detection means to sense the presence of a vehicle in a zone disposed in one of said streets to produce a call indication when a vehicle is present in said zone and a no-call indication when no vehicle is present, and

C. a delay unit coupling said detection means to said controller and responsive to said indications, said unit including first means to defer the transfer of the call indication to said controller for an adjustable period to permit a sensed vehicle to make a right turn on a red signal, and second means independent of the first means to defer the transfer of a no-call indication to said controller for an adjustable period to maintain a green signal after a first vehicle has departed the zone and a second vehicle spaced from said first vehicle advances toward said zone.

2. A system as set forth in claim 1, wherein said detection means includes an inductive loop buried below said street.

3. A system as set forth in claim 1, wherein said unit includes a flip-flop circuit constituted by two crosscoupled main transistors, an output relay connected in the collector circuit of one of said transistors and having a normally-closed switch coupled to said controller to produce a green signal when the relay is de-energized, and first and second control means coupled to said first and second main transistors and incorporating an adjustable delay network, said first control means being responsive to a no-call indication to render said first transistor non-conductive after a delay period to cause said switch to open and to cause said controller to produce a red signal, said second control means being responsive to a call indication to render said second transistor non-conductive after a delay period, thereby to render said first transistor conductive to cause said switch to open to produce a red signal.

4. A system as set forth in claim 3, wherein each control means is constituted by a unijunction transistor having a charging capacitor in the emitter circuit thereof which, when the capacitor attains a given charge, fires the unijunction transistor to produce a pulse rendering the associated main transistor non-conductive. 

1. A traffic-control system comprising A. a traffic-controller adapted to operate green and red signals at an intersection of two streets, B. detection means to sense the presence of a vehicle in a zone disposed in one of said streets to produce a call indication when a vehicle is present in said zone and a no-call indication when no vehicle is present, and C. a delay unit coupling said detection means To said controller and responsive to said indications, said unit including first means to defer the transfer of the call indication to said controller for an adjustable period to permit a sensed vehicle to make a right turn on a red signal, and second means independent of the first means to defer the transfer of a nocall indication to said controller for an adjustable period to maintain a green signal after a first vehicle has departed the zone and a second vehicle spaced from said first vehicle advances toward said zone.
 2. A system as set forth in claim 1, wherein said detection means includes an inductive loop buried below said street.
 3. A system as set forth in claim 1, wherein said unit includes a flip-flop circuit constituted by two cross-coupled main transistors, an output relay connected in the collector circuit of one of said transistors and having a normally-closed switch coupled to said controller to produce a green signal when the relay is de-energized, and first and second control means coupled to said first and second main transistors and incorporating an adjustable delay network, said first control means being responsive to a no-call indication to render said first transistor non-conductive after a delay period to cause said switch to open and to cause said controller to produce a red signal, said second control means being responsive to a call indication to render said second transistor non-conductive after a delay period, thereby to render said first transistor conductive to cause said switch to open to produce a red signal.
 4. A system as set forth in claim 3, wherein each control means is constituted by a unijunction transistor having a charging capacitor in the emitter circuit thereof which, when the capacitor attains a given charge, fires the unijunction transistor to produce a pulse rendering the associated main transistor non-conductive. 